An important factor in the use of lasers is the precision with which the beam is controlled.
Various sensors are used to measure the beam location, usually by measuring variations in beam intensity from a null state, to provide outputs to a control system to adjust the beam alignment.
The prior art includes a variety of different laser control methods including radiometers (Clifford 3,348,047), optics (Kunitsugo 4,867,560), radar (Lloyd 4,738,531), florescence (McClung, Jr. 4,700,068), and mirrors (Anderson 3,579,140). The sensor of choice is usually some type of photoconductive device.
More particularly, U.S. Pat. No. 3,723,013 discloses four quadrant silicon photodetectors used to control the positioning of a laser beam. U.S. Pat. No. 4,243,888 teaches the use of a silicon disk having thermoelectric properties. The reference discloses the absorption of the beam energy by the silicon detector can be controlled by introducing a selected amount of impurities into the silicon. U.S. Pat. No. 4,035,654 teaches the use of a photoconductive sensor wherein a disk, which can be silicon, germanium or zinc selenide, is used as the sensor in the UV range.
In the prior art, the photoconductive sensors are typically specific for the laser beam to be controlled. The present invention is directed to modifying a semiconductor material such as germanium, silicon or silicon carbide to make it specific for a wavelength range it is not normally specific for.
Broadly the invention comprises a doped semiconductor which functions as a photoconductor and is responsive to wavelengths it would not be responsive to in its undoped state. The dopant provides an acceptor or donor state whether functioning as a P-type or N-type semiconductor which has an acceptor level corresponding generally to the energy of the laser light. When the doped semiconductor is exposed to radiation, whose frequency is above a certain threshold, the incoming radiation transfers energy to an electron which then takes on a new energy state in the conduction band and contributes to the electrical conductivity. That is, its resistance decreases and hence its conductivity increases and this change is measured.
In the preferred embodiment of the invention, silver is used to provide an acceptor site in a germanium crystal when CO.sub.2 laser light excites the crystal. The germanium is formed in a segmented circular array, with the inwardly directed edges of the segments defining an aperture.
In a preferred embodiment, the invention comprises a photoconductive sensor which includes a doped semiconductor, the dopant changing the conductive characteristics of the semiconductor. An acceptor or a donor site is spaced apart from a band edge in an amount responsive to the power of an incident beam of energy.